This invention relates to wireless communication systems and, more particularly, to wireless communication systems using multiple antennas at the transmitter and/or multiple antennas at the receiver.
Wireless communication systems that use multiple antennas at the transmitter and optionally multiple antennas at the receiver, so-called multiple-input and/or multiple-output systems, respectively, can achieve dramatically improved capacity compared to single antenna systems, i.e. single antenna to single antenna systems. In random scattering environments increasing the number of antennas at the receiver or at the transmitter (or both) produces a greater capacity.
In multiple-input systems, a primitive data stream—the bits to be transmitted to a particular terminal—is divided into a plurality of sub-streams, each of which is processed, typically by encoding it and modulating it onto a carrier signal. The processed sub-streams are then transmitted. At any particular time, each processed sub-stream is transmitted over a different transmit antenna than the other processed sub-streams.
The transmission paths between the transmit and receive antennas are typically referred to as channels. There is a channel between each transmit and each receive antenna. Each channel has its own channel characteristic.
The signals emanating from the transmit antennas arrive at the receive antennas. Thus, the received signal at each of the receive antennas is typically a superposition of each of the transmitted signals as modified by the channel characteristics. Though the transmitted signals interfere with each other, received signals can be processed to separate the transmitted signals from one another. The separated signals are then decoded to recover the respective sub-streams.
In particular, even if the channel characteristics are not known, the coding and modulation schemes used to process the sub-streams can nonetheless be used to separate the transmitted signals. The use of coding and modulation schemes to separate the transmitted signals is commonly referred to as non-coherent demodulation. In this situation, however, separating out the transmitted signals so that respective sub-streams can be decoded with acceptable packet error rates typically requires transmitting at lower data rates than if the channel characteristics were known.
The channel characteristics may be determined during a training phase during which known symbol sequences, which are referred to as training sequences, are transmitted on each transmit antenna. The essential characteristics of the training sequences are provided to the receiver and transmitter. The receiver processes received training sequences to produce accurate estimates of the channel characteristics between the transmit and receive antennas.
The channel characteristics change over time and, therefore, there is typically a training phase at the start of each transmission burst. Because the training sequences increase the duration of the bursts without increasing their information content, the training sequences reduce the data rate. Thus, it is desirable to keep the duration of the training phase as short as possible. Furthermore, in order to keep the training phase as short as possible, it is also desirable to transmit training sequences concurrently and not sequentially. However, if the training sequences are transmitted concurrently they interfere with each other because the receive antennas receive a superposition of the training sequences. To reduce such interference, the concurrently transmitted training sequences are orthogonal to each other.